Corn Rootworms
Among the corn rootworms are found some of the most destructive insect pests. The corn rootworms Diabrotica virgifera virgifera (western corn rootworm) and Diabrotica barberi (northern corn rootworm) are considered the most serious inset pests of corn in the major corn-producing states of the USA and Canada (Levine and Oloumi-Sadeghi, 1991, Annu. Rev. Entomol. 36, 229-55). The larvae feed on corn roots, thus causing direct damage to corn growth and corn yields. Extensive root damage makes plants also more susceptible to lodging. Adults are strong flyers and feed on corn silks and leaves. Furthermore, these insects are also vectors of diseases such as the maize chlorotic mottle virus. Costs for soil insecticides to control larval damage to the root systems of corn and aerial sprays to reduce beetle damage to corn silks, when combined with crop losses, are estimated to reach up to 1 billion US dollars annually (Metcalf, 1986, Foreword in "Methods for the study of Pest Diabrotica", pp. vii-xv, eds. Krysan J. L. and Miller, T. A., Springer Verlag, New York). Besides corn, Diabrotica species also cause major damage to vegetable plants, mainly to plants of the Cucurbitaceae.
Although the CryIIIA protein was found to be very toxic to the Colorado potato beetle, the protein has been found to have weak, if any, toxicity to Diabrotica spp. (Johnson et al., 1993, J. Econ. Entomol. 86, 330). According to Slaney et al. (1992, Insect Biochem. Molec. Biol. 22, 9-18) the toxin encoded by the cryIIIA gene was found to be at least 2000 times less effective to Diabrotica undecimpunctata howardi larvae than to the Colorado potato beetle, Leptinotarsa decemlineata. Some mortality of crude extracts comprising the Bt tenebrionis toxin, although at undetermined concentration, to western corn rootworm has been reported (EP 0318143).